Seachoice Phillips Machine Screw - Flat Head, 5/16"-18 x 5" - 00317. Reliable Fastening with a Low-Profile Finish. The Seachoice Phillips Machine Screw - Flat Head, 5/16"-18 x 5" - 00317 is built to deliver solid fastening with a clean, flush finish. Whether you're assembling structural components or replacing hardware in marine or industrial settings, this long 5-inch stainless steel screw provides the strength and corrosion resistance you need.. With its flat head design and Phillips drive, th